In densely deployed small cells, low interference between cells facilitates efficient operation. One mechanism that has been introduced to provide low interference between cells is small cell on/off, which can also provide energy savings. In small cell on/off, the small cell may be turned on and off where the “on” and “off” period may depend on various criteria and/or the application.
There are two kinds of small cell on/off: semi-static small cell on/off and dynamic small cell on/off. In semi-static small cell on/off, the criteria for cell on/off can be traffic load, UE arrival/departure, etc. In dynamic small cell on/off, the small cell can be turned on and off on the radio frame or subframe level. The criteria for dynamic small cell on/off can be packet arrival/completion or interference coordination and avoidance (i.e., reduce interference towards other nodes or user equipment (UEs)). Thus, the cell turns off at the subframe boundary (or end of the current subframe) when the transmission of a packet is completed, and turns on at the next subframe boundary where a packet arrives. Another purpose of small cell on/off can be for energy saving.
There are different operational modes of small cell on/off. A first example is handover or cell change mode. In this mode, a UE in CONNECTED mode is always attached to a cell. The network may decide to offload the UE by handover to a small cell due to, for example, increased traffic demand. The small cell that can be “off” wakes up to serve the UE. The handover time in this case depends on the backhaul delay and the handover execution time. After completion of the transmission and/or reception of data, the UE goes to IDLE mode or is handed over to another cell. The small cell can then be turned off Another example of cell change is RRC connection release with re-direction.
A second example operating mode is SCell only mode. In SCell only mode, a carrier aggregation (CA) capable UE is connected to a Primary Cell (PCell), and the network configures a secondary cell (SCell) that can be turned on or off. If the network decides to offload the UE traffic to SCell, then the SCell is turned on. A third example is dual connectivity mode. In this mode, the UE is connected to two network nodes. One of the nodes to which the UE is connected can be turned on and off. A fourth example is serving cell mode. In serving cell mode, a cell can be either on or off when a UE is connected to it. Certain procedures for radio resource management (RRM), radio link monitoring (RLM), and channel state information (CSI) measurements must be designed for this case.
In small cell on/off, the eNB can be off for long periods of time. To assist the UE with measurements, a discovery signal can be used. The discovery signal should support properties for enabling RRM measurements, RLM related procedures, and coarse time/frequency synchronization. In order to make the measurements possible, the eNB has to wake up periodically (e.g., once every 40 ms, 80 ms, or 160 ms, etc.) and send the discovery signal so that it can be used by the UE for mobility related operations such as cell identification, RLM, and measurement. So that the UE can perform measurements for RRM, a discovery reference signal (DRS) is provided by the network with a configurable structure. The UE performs measurements both on the serving cell as well as neighboring cells within the configured time intervals of the discovery burst.
Since the discovery signal is rather sparse in time, it is desirable that the UE be able to make a meaningful measurement in one instance of the discovery signal rather than having to wait for multiple instances, which may occur tens or hundreds of milliseconds apart. In addition, in order to make the measurement based on fewer samples in time more reliable, the discovery signal may need to be sent on wide bandwidth (e.g., the whole bandwidth).
It has been agreed in 3GPP that a UE assumes primary synchronization signals, secondary synchronization signals, and common reference signals (PSS/SSS/CRS) in the DRS. Additionally, CSI-RS is assumed in the DRS for measurement if configured by higher layers. Both CRS-based reference signal received power (RSRP) measurements and CSI-RS-based RSRP measurements are supported. The UE may report DRS-based RSRP and reference signal received quality (RSRQ), as well as associated physical cell ID (PCID) and information for TP identification.
For DRS-based measurement, a UE assumes that a DRS occasion for a cell consists of one instance of PSS/SSS according to Rel-8. The UE also assumes that CRS is transmitted at least in the same subframe(s) as PSS/SSS, and that a DRS occasion can include multiple CSI-RS RE configurations. The different CSI-RS configurations may be in the same or different subframe(s). A DRS occasion for a cell includes N consecutive subframes (N<=5), and a DRS occasion for a cell is transmitted every M ms (candidate values for M so far are 40, 80, 160).
Several radio-related measurements may be used by the UE or the network node to establish and keep the connection, as well as ensuring the quality of a radio link. The measurements are used in radio resource control (RRC) idle state operations such as cell selection, cell reselection (e.g., between E-UTRANs, between different radio access technologies (RATs), and to non-3GPP RATs), minimization of drive test (MDT), and also in RRC connected state operations such as cell change (e.g., handover between E-UTRANs, handover between different RATs, and handover to non-3GPP RATs). The UE has to first detect a cell, and therefore cell identification (e.g., acquisition of a physical cell identity (PCI)), is also a signal measurement. The UE may also have to acquire the cell global ID (CGI) of a UE. The RSRP and RSRQ are used for at least RRM, such as for mobility, which includes mobility in RRC connected state as well as in RRC idle state. The RSRP and RSRQ are also used for other purposes such as for enhanced cell ID positioning, minimization of drive test etc.
In RRC connected state, the UE can perform intra-frequency measurements without measurement gaps. As a general rule, however, the UE performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements for the UE requiring gaps, the network has to configure the measurement gaps. Two periodic measurement gap patterns—both with a measurement gap length of 6 ms—are defined for LTE: measurement gap pattern #0 with repetition period 40 ms; and measurement gap pattern #1 with repetition period 80 ms. The measurements performed by the UE are then reported to the network, which may use them for various tasks.
The radio network node (e.g., base station) may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between UE and itself, UL SINR, UL SNR, UL signal strength, Received Interference Power (RIP), etc. The eNB may also perform positioning measurements.
The UE also performs measurements on the serving cell (also referred to as primary cell) in order to monitor the serving cell performance. This is known as radio link monitoring (RLM) or RLM related measurements in LTE. For RLM, the UE monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving or PCell. The UE detects out of sync and in sync by comparing the estimated quality with the thresholds Qout and Qin respectively. The threshold Qout and Qin are defined as the level at which the downlink radio link cannot be reliably received, and corresponds to 10% and 2% block error rate of a hypothetical PDCCH transmissions, respectively. The overall serving cell or neighbour cell measurement quantity results include non-coherent averaging of two or more basic non-coherent averaged samples.
FIG. 1 is a schematic diagram of an example averaging of RSRP measurements in E-UTRAN. More particularly, FIG. 1 illustrates an example in which the UE obtains the overall measurement quantity result by collecting four non-coherent averaged samples or snapshots 5A-5D (each of 3 ms length in this example) during the physical layer measurement period (i.e., 200 ms) when no discontinuous reception (DRX) is used or when the DRX cycle is not larger than 40 ms. Every coherent averaged sample is 1 ms long. The sampling also depends upon the length of the DRX cycle. For example, for DRX cycle greater than 40 ms, the UE typically takes one sample every DRX cycle over the measurement period. A similar measurement sampling mechanism is used for other signal measurements by the UE and also by the BS for UL measurements. Although FIG. 1 illustrates a particular example averaging of RSRP measurements, the exact sampling may vary according to particular implementations.